Do a bunch like that, and then come back later to factor them. See if you can factor {this example} back into (x-4)(x+5)

If you get stuck, you can go back to the work where you generated the problem to begin with; it will have a roadmap to the result you want.

(It is possible that you made a mistake in multiplying them originally, so if one side looks right, check the other side)

A lot of insight into math can be obtained this way - start from the answer (or at least the form of the answer) and see how that turns into the question. Then go backwards.

Jose

Order of the Sillies, Honoris Causam - bestowed by charlie_grumbles on NP 859 * OTTscar winner: Wordsmith - bestowed by yappobiscuts and the OTT on NP 1832 * Ecclesiastical Calendar of the Order of the Holy Contradiction * Please help addams if you can. She needs all of us.

I second ucim's recommendation of making up your own problems. Going through this process and getting familiar with it can really help your understanding of *why* the methods used to solve the original factoring problem work the way they do.